It is well known that a waterjet generated by directing pressurized water through a suitable orifice at a work-piece can remove a variety of organic and inorganic surface coatings such as paint or deposits. Modern cleaning systems often use waterjets to remove rust, scale or a variety of coatings. Much research has been conducted regarding the use of high and ultra-high pressure waterjets for cleaning applications. This has led to variations in waterjet technology directed at amplifying the impact of the water against the work-piece to increase material removal rates. A key component for successful waterjet cleaning/removal processes is the type of nozzle selected. Variations that have been investigated include: abrasive carried in a fluid, e.g. abrasive jet; interrupting the flow of exiting fluid, e.g. pulse waterjet; and forming bubbles within the fluid, e.g. cavitation waterjet.
The use of a pressurized fluid to carry abrasives to the work-piece is well known and commonly used. Various types of abrasives such as sand, pumice, walnut shells and glass beads have been added to pressurized fluids to create abrasive jets. Abrasive jets have the capability of eroding most surfaces and are most commonly used in situations where fluid jets without abrasive would not effectively clean the intended surface.
However, it is often undesirable to use an abrasive carried in a fluid. Many suitable abrasives are in the form of fine powders or dry granules that can cake up when wetted by a fluid, thus blocking the nozzle. Additionally, the erosive nature of abrasives cause nozzle wear and may contaminate or damage the surfaces being cleaned. The abrasive also creates additional hazardous waste requiring disposal.
Pulsed waterjets are also well known in the prior art. Pulsed waterjets are typically classified as either natural or forced pulse waterjets. There are several techniques for producing natural and forced pulse waterjets.
Natural pulsed waterjets, e.g. fanjets, are created when a continuous stream of high pressure water is directed though an orifice. Due to friction the emerging stream of water naturally breaks up into droplets at some distance from the nozzle. The droplets are considered to be natural pulsed waterjets. Natural pulsed waterjets are very common, and are suitable for simple cleaning operations.
Forced pulse waterjets are typically created by use of mechanical or electromechanical internally mounted devices used to interrupt the flow of water emerging from the nozzle. Devices such as a rotor modulating a continuous stream at a predetermined frequency, or an ultrasonic transformer or magnetostrictive transducer may be placed within the nozzle for causing pulses within the emerging waterjet. The pulses begin to disintegrate into droplets soon after the waterjet emerges from the nozzle.
Forced pulse waterjets are more effective than natural pulsed waterjets and can be used for removal or erosion of harder coatings. However, these devices require expensive and complex electrical components in order to actually produce an effective amount of pulses. Complex components often reduce reliability and increase the cost of manufacturing and maintaining these devices.
Cavitation waterjets are also well known in the art. The principle of cavitation involves directing water past a body. As the static pressure decreases below the vapor pressure of the water, bubbles of vapor form within the waterjet. Since the region of low pressure is generally small, the bubbles burst soon after they leave that region. When the nozzle is located at an optimum distance from the work-piece the bubbles implode upon themselves as they strike the surface. The imploding bubbles generate high forces and aid in cleaning or eroding surfaces. Cavitation jets are typically effective for cleaning and removing a wide variety of surfaces coatings.
In prior devices cavitation bubbles have been formed by directing high pressure water past a pin member located within the nozzle, by turning vanes to induce vortex cavitation, and by directing high pressure water past sharp corners within a nozzle causing pressure differentials. Unfortunately, for cavitating nozzles to actually produce a substantial amount of bubbles, the prior art requires nozzle members to be accurately machined and positioned. Precision machining and assembly often results in high production costs. Additionally, natural wear and lack of maintenance during use often results in nozzles being less efficient than desired.
Accordingly, a waterjet nozzle capable of producing a high impact waterjet when supplied with a source of ultra-high pressure liquid, e.g. water, would satisfy a long felt need in the art.